package PPT3;

class Useful{
    public void f(){

    }
    public void g(){

    }
}
class MoreUseful extends Useful{
    @Override
    public void f() {

    }
    public void g(){

    }
    public void u(){

    }
    public void v(){

    }
    public void w(){

    }
}
public class RTTI {
    public static void main(String[] args){
        Useful[]x ={new Useful(),new MoreUseful()};
        x[0].f();
        x[1].g();
        //Compile time:method not found in Useful:
        //! x[1].u();
        ((MoreUseful)x[1]).u();//Downcast/RTTI
        ((MoreUseful)x[0]).u();//抛出异常
    }
}
